The invention concerns a method for the manufacture of hot strips or heavy plates from a denitrated steel.
For a long time the demand continued for the development of higher strength steels with high toughness values at low temperature, in the form of hot strips or heavy plates. These are used, for example, in large diameter long-distance pipelines. Controlled hot rolling has been used more and more as an economical method for the production of thermo-mechanically treated hot strips or heavy plates. As part of a thermo-mechanical treatment for steels it is understood to effect a controlled transformation of the steel in a temperature range around the transformation point A.sub.r.sbsb.3 with a simultaneously controlled cooling and/or transformation of the structure.
It is known to use denitrated steel with a composition of 0.04 to 0.16% carbon, 1.25 to 1.90% manganese, 0.02 to 0.55% silicon, 0.004 to 0.020% phosphorus, 0.002 to 0.015% sulfur, 0.02 to 0.08% aluminum, 0.2 to 0.08% niobium, the remainder being iron and possibly contaminants. The steel can also be alloyed with the addition of 0.015 to 0.35% molybdenum, 0.10 to 0.30% chromium and/or 0.30 to 0.90% nickel, alone or in combination.
During the mechanical-technological testing of these steels, particularly with the presence of notches and over a wide temperature range, one frequently observes splits perpendicular to the fracture surface in the upper part of the complete brittle failure stage. These are designated "separations", "cleavage" or "splitting". This tendency of splitting at the fracture surface of thermo-mechanically treated steel is, for example, of significance in the operation of large diameter long-distance pipelines, since the capacity of these steels to stop fracture propagation is thereby reduced.
Proposals have already been made for the production of higher strength steels for use in large diameter long-distance pipelines in which splitting at the fracture during the notch impact testing is no longer found. However, high alloying and manufacturing costs are connected with all of them. For example, it is recommended in De-OS No. 26 53 847 to alloy the steel by up to 3.5% or 2.5% addition of chromium and manganese, after the steel has been subjected to a nitrogen enrichment up to a content of 0.012%. Furthermore the hot rolling of this steel is complicated. The rolled stock will be subjected to a deformation from 30 to 60% at temperatures between 950.degree. C. and 1100.degree. C., a subsequent prescribed interruption of hot rolling and a deformation of 75 to 95% of the original thickness at temperatures between 700.degree. C. and 900.degree. C. The deformed structure will finally be converted into the lower bainite stage. The alloying of the chromium and manganese additions raises the price of the known steels considerably. On account of the complicated and expensive rolling operation further increased manufacturing costs arise.